1. Field of the Invention
The present invention relates in general to exhaust control systems of an internal combustion engine, and more particularly, to exhaust control systems of a type which is designed to reduce the noise of the combustion and exhaust of the engine.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional exhaust control system of the above-mentioned type will be outlined with reference to FIG. 4 of the accompanying drawings, which is disclosed in Japanese Utility Model First Provisional Publication No. 61-118914.
As is seen from the drawing, the system comprises an electrically controlled valve 101 which is installed in one of tail pipes 100a and 100b of a muffler 100 to which an exhaust tube 104 extending from an internal combustion engine (not shown) is connected. The valve 101 is controlled by a control device 16 which comprises an engine speed sensor 108, a control circuit 110 and an actuator 112. That is, when the engine speed sensor 108 senses that the engine speed exceeds a predetermined level (for example, 2500 rpm), the control circuit 110 causes the actuator 112 to open the valve 102. With this, output of the engine is increased. While, when the engine speed is sensed under the given degree, the valve 102 is closed to improve the noise muffling effect of the muffler 100.
However, due to its inherent construction, the conventional system has the following drawback.
That is, the characteristic of the exhaust sound pressure varies depending on the temperature of the exhaust gas. This will be understood from the graph of FIG. 3 in which the exhaust sound pressure at low temperature is depicted by a broken line, while, the sound pressure at high temperature is by a solid line. From this graph, it will be appreciated that with increase in temperature of the exhaust gas, the same sound pressure level of the exhaust gas shifts toward a higher engine speed side. That is, the same sound pressure level "P" of the exhaust gas appears at the engine speed "M" when the gas temperature is low, but appears at the higher engine speed "N" when the gas temperature is high.
Since the conventional exhaust control system is constructed without respect to the above-mentioned exhaust sound pressure characteristic, the same has failed to exhibit a satisfied performance.